<!DOCTYPE html>
<html lang="zh-CN">
<head>
  <meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=2">
<meta name="theme-color" content="#222">
<meta name="generator" content="Hexo 5.2.0">
  <link rel="apple-touch-icon" sizes="180x180" href="/images/apple-touch-icon-next.png">
  <link rel="icon" type="image/png" sizes="32x32" href="/images/favicon-32x32-next.png">
  <link rel="icon" type="image/png" sizes="16x16" href="/images/favicon-16x16-next.png">
  <link rel="mask-icon" href="/images/logo.svg" color="#222">

<link rel="stylesheet" href="/css/main.css">


<link rel="stylesheet" href="/lib/font-awesome/css/all.min.css">

<script id="hexo-configurations">
    var NexT = window.NexT || {};
    var CONFIG = {"hostname":"llmysnow.top","root":"/","scheme":"Gemini","version":"7.8.0","exturl":false,"sidebar":{"position":"left","display":"post","padding":18,"offset":12,"onmobile":false},"copycode":{"enable":true,"show_result":true,"style":null},"back2top":{"enable":true,"sidebar":false,"scrollpercent":true},"bookmark":{"enable":false,"color":"#222","save":"auto"},"fancybox":false,"mediumzoom":false,"lazyload":true,"pangu":false,"comments":{"style":"tabs","active":null,"storage":true,"lazyload":false,"nav":null},"algolia":{"hits":{"per_page":10},"labels":{"input_placeholder":"Search for Posts","hits_empty":"We didn't find any results for the search: ${query}","hits_stats":"${hits} results found in ${time} ms"}},"localsearch":{"enable":true,"trigger":"auto","top_n_per_article":1,"unescape":false,"preload":false},"motion":{"enable":true,"async":false,"transition":{"post_block":"fadeIn","post_header":"slideDownIn","post_body":"slideDownIn","coll_header":"slideLeftIn","sidebar":"slideUpIn"}},"path":"search.xml"};
  </script>

  <meta name="description" content="当浏览器开辟出供 js 执行的栈内存之后，代码并不是立即自上而下执行，而是需要先做一些事情：把当前作用域中带 var 和 function 的关键字进行提前的声明和定义（变量提升）  var：只声明，未定义（不赋值） function：声明和定义（赋值）一起完成">
<meta property="og:type" content="article">
<meta property="og:title" content="JS 变量提升">
<meta property="og:url" content="http://llmysnow.top/posts/ea26882a/index.html">
<meta property="og:site_name" content="奇幻的梦之旅">
<meta property="og:description" content="当浏览器开辟出供 js 执行的栈内存之后，代码并不是立即自上而下执行，而是需要先做一些事情：把当前作用域中带 var 和 function 的关键字进行提前的声明和定义（变量提升）  var：只声明，未定义（不赋值） function：声明和定义（赋值）一起完成">
<meta property="og:locale" content="zh_CN">
<meta property="og:image" content="https://gitee.com/lilyn/pic/raw/master/js-img/JavaScript%E8%BF%90%E7%AE%97%E7%AC%A6%E4%BC%98%E5%85%88%E7%BA%A7.jpg">
<meta property="article:published_time" content="2020-11-24T12:00:24.088Z">
<meta property="article:modified_time" content="2021-05-28T01:44:29.935Z">
<meta property="article:author" content="LiLyn">
<meta property="article:tag" content="JavaScript">
<meta property="article:tag" content="前端">
<meta property="article:tag" content="作用域">
<meta property="article:tag" content="变量提升特殊性">
<meta name="twitter:card" content="summary">
<meta name="twitter:image" content="https://gitee.com/lilyn/pic/raw/master/js-img/JavaScript%E8%BF%90%E7%AE%97%E7%AC%A6%E4%BC%98%E5%85%88%E7%BA%A7.jpg">

<link rel="canonical" href="http://llmysnow.top/posts/ea26882a/">


<script id="page-configurations">
  // https://hexo.io/docs/variables.html
  CONFIG.page = {
    sidebar: "",
    isHome : false,
    isPost : true,
    lang   : 'zh-CN'
  };
</script>

  <title>JS 变量提升 | 奇幻的梦之旅</title>
  






  <noscript>
  <style>
  .use-motion .brand,
  .use-motion .menu-item,
  .sidebar-inner,
  .use-motion .post-block,
  .use-motion .pagination,
  .use-motion .comments,
  .use-motion .post-header,
  .use-motion .post-body,
  .use-motion .collection-header { opacity: initial; }

  .use-motion .site-title,
  .use-motion .site-subtitle {
    opacity: initial;
    top: initial;
  }

  .use-motion .logo-line-before i { left: initial; }
  .use-motion .logo-line-after i { right: initial; }
  </style>
</noscript>

<link rel="alternate" href="/atom.xml" title="奇幻的梦之旅" type="application/atom+xml">
</head>
<!--即时预加载技术，在用户点击之前预先加载页面-->
<script src="https://cdn.jsdelivr.net/gh/TRHX/CDN-for-itrhx.com@2.0.2/js/instantclick-1.2.2.js" type="module"></script>
<body itemscope itemtype="http://schema.org/WebPage">
  <div class="container use-motion">
    <div class="headband"></div>
	  <!--<a target="_blank" rel="noopener" href="https://github.com/llwodexue" class="github-corner" aria-label="View source on GitHub"><svg width="80" height="80" viewBox="0 0 250 250" style="fill:#151513; color:#fff; position: absolute; top: 0; border: 0; right: 0;" aria-hidden="true"><path d="M0,0 L115,115 L130,115 L142,142 L250,250 L250,0 Z"></path><path d="M128.3,109.0 C113.8,99.7 119.0,89.6 119.0,89.6 C122.0,82.7 120.5,78.6 120.5,78.6 C119.2,72.0 123.4,76.3 123.4,76.3 C127.3,80.9 125.5,87.3 125.5,87.3 C122.9,97.6 130.6,101.9 134.4,103.2" fill="currentColor" style="transform-origin: 130px 106px;" class="octo-arm"></path><path d="M115.0,115.0 C114.9,115.1 118.7,116.5 119.8,115.4 L133.7,101.6 C136.9,99.2 139.9,98.4 142.2,98.6 C133.8,88.0 127.5,74.4 143.8,58.0 C148.5,53.4 154.0,51.2 159.7,51.0 C160.3,49.4 163.2,43.6 171.4,40.1 C171.4,40.1 176.1,42.5 178.8,56.2 C183.1,58.6 187.2,61.8 190.9,65.4 C194.5,69.0 197.7,73.2 200.1,77.6 C213.8,80.2 216.3,84.9 216.3,84.9 C212.7,93.1 206.9,96.0 205.4,96.6 C205.1,102.4 203.0,107.8 198.3,112.5 C181.9,128.9 168.3,122.5 157.7,114.1 C157.9,116.9 156.7,120.9 152.7,124.9 L141.0,136.5 C139.8,137.7 141.6,141.9 141.8,141.8 Z" fill="currentColor" class="octo-body"></path></svg></a><style>.github-corner:hover .octo-arm{animation:octocat-wave 560ms ease-in-out}@keyframes octocat-wave{0%,100%{transform:rotate(0)}20%,60%{transform:rotate(-25deg)}40%,80%{transform:rotate(10deg)}}@media (max-width:500px){.github-corner:hover .octo-arm{animation:none}.github-corner .octo-arm{animation:octocat-wave 560ms ease-in-out}}</style>-->
    <header class="header" itemscope itemtype="http://schema.org/WPHeader">
      <div class="header-inner"><div class="site-brand-container">
  <div class="site-nav-toggle">
    <div class="toggle" aria-label="切换导航栏">
      <span class="toggle-line toggle-line-first"></span>
      <span class="toggle-line toggle-line-middle"></span>
      <span class="toggle-line toggle-line-last"></span>
    </div>
  </div>

  <div class="site-meta">

    <a href="/" class="brand" rel="start">
      <span class="logo-line-before"><i></i></span>
      <h1 class="site-title">奇幻的梦之旅</h1>
      <span class="logo-line-after"><i></i></span>
    </a>
      <p class="site-subtitle" itemprop="description">http://llmysnow.top/</p>
  </div>

  <div class="site-nav-right">
    <div class="toggle popup-trigger">
        <i class="fa fa-search fa-fw fa-lg"></i>
    </div>
  </div>
</div>




<nav class="site-nav">
  <ul id="menu" class="main-menu menu">
        <li class="menu-item menu-item-home">

    <a href="/" rel="section"><i class="fa fa-home fa-fw"></i>首页</a>

  </li>
        <li class="menu-item menu-item-tags">

    <a href="/tags/" rel="section"><i class="fa fa-tags fa-fw"></i>标签</a>

  </li>
        <li class="menu-item menu-item-categories">

    <a href="/categories/" rel="section"><i class="fa fa-th fa-fw"></i>分类</a>

  </li>
        <li class="menu-item menu-item-archives">

    <a href="/archives/" rel="section"><i class="fa fa-archive fa-fw"></i>归档</a>

  </li>
      <li class="menu-item menu-item-search">
        <a role="button" class="popup-trigger"><i class="fa fa-search fa-fw"></i>搜索
        </a>
      </li>
  </ul>
</nav>



  <div class="search-pop-overlay">
    <div class="popup search-popup">
        <div class="search-header">
  <span class="search-icon">
    <i class="fa fa-search"></i>
  </span>
  <div class="search-input-container">
    <input autocomplete="off" autocapitalize="off"
           placeholder="搜索..." spellcheck="false"
           type="search" class="search-input">
  </div>
  <span class="popup-btn-close">
    <i class="fa fa-times-circle"></i>
  </span>
</div>
<div id="search-result">
  <div id="no-result">
    <i class="fa fa-spinner fa-pulse fa-5x fa-fw"></i>
  </div>
</div>

    </div>
  </div>

</div>
    </header>

    
  <div class="back-to-top">
    <i class="fa fa-arrow-up"></i>
    <span>0%</span>
  </div>


    <main class="main">
      <div class="main-inner">
        <div class="content-wrap">
          

          <div class="content post posts-expand">
            

    
  
  
  <article itemscope itemtype="http://schema.org/Article" class="post-block" lang="zh-CN">
    <link itemprop="mainEntityOfPage" href="http://llmysnow.top/posts/ea26882a/">

    <span hidden itemprop="author" itemscope itemtype="http://schema.org/Person">
      <meta itemprop="image" content="/images/logo.jpg">
      <meta itemprop="name" content="LiLyn">
      <meta itemprop="description" content="">
    </span>

    <span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization">
      <meta itemprop="name" content="奇幻的梦之旅">
    </span>
      <header class="post-header">
        <h1 class="post-title" itemprop="name headline">
          JS 变量提升
        </h1>

        <div class="post-meta">
            <span class="post-meta-item">
              <span class="post-meta-item-icon">
                <i class="far fa-calendar"></i>
              </span>
              <span class="post-meta-item-text">发表于</span>

              <time title="创建时间：2020-11-24 20:00:24" itemprop="dateCreated datePublished" datetime="2020-11-24T20:00:24+08:00">2020-11-24</time>
            </span>
            <span class="post-meta-item">
              <span class="post-meta-item-icon">
                <i class="far fa-folder"></i>
              </span>
              <span class="post-meta-item-text">分类于</span>
                <span itemprop="about" itemscope itemtype="http://schema.org/Thing">
                  <a href="/categories/JavaScript/" itemprop="url" rel="index"><span itemprop="name">JavaScript</span></a>
                </span>
            </span>

          <br>
            <span class="post-meta-item" title="本文字数">
              <span class="post-meta-item-icon">
                <i class="far fa-file-word"></i>
              </span>
                <span class="post-meta-item-text">本文字数：</span>
              <span>4.8k</span>
            </span>
            <span class="post-meta-item" title="阅读时长">
              <span class="post-meta-item-icon">
                <i class="far fa-clock"></i>
              </span>
                <span class="post-meta-item-text">阅读时长 &asymp;</span>
              <span>4 分钟</span>
            </span>

        </div>
      </header>

    
    
    
    <div class="post-body" itemprop="articleBody">

      
        <p>当浏览器开辟出供 js 执行的栈内存之后，代码并不是立即自上而下执行，而是需要先做一些事情：把当前作用域中带 var 和 function 的关键字进行提前的声明和定义（变量提升）</p>
<ul>
<li>var：只声明，未定义（不赋值）</li>
<li>function：声明和定义（赋值）一起完成</li>
</ul>
<a id="more"></a>

<h2 id="匿名函数具名化"><a href="#匿名函数具名化" class="headerlink" title="匿名函数具名化"></a>匿名函数具名化</h2><p>匿名函数具名化【规范函数执行的顺序】</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> fn = <span class="function"><span class="keyword">function</span> <span class="title">sum</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">&#x27;ok&#x27;</span>)</span><br><span class="line">  sum = <span class="number">100</span> <span class="comment">// 默认情况下，其值是不能修改的</span></span><br><span class="line">  <span class="built_in">console</span>.log(sum) <span class="comment">// 函数本身 // 具名话的名字可以在函数内部的上下文中使用【代表函数本身】</span></span><br><span class="line"></span><br><span class="line">  <span class="built_in">console</span>.log(sum) <span class="comment">// undefined // 虽然具名化的名字会在当前函数的私有上下文中被声明，当做私有变量处理，但是它的优先级是比较低的</span></span><br><span class="line">  <span class="keyword">var</span> sum</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">sum() <span class="comment">// 具名化设置的函数名不能在函数以外使用【因为没有在当前上下文中声明这个变量】</span></span><br></pre></td></tr></table></figure>

<p>匿名函数具名化可以让匿名函数实现递归调用</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&#x27;use strict&#x27;</span></span><br><span class="line">;(<span class="function"><span class="keyword">function</span> <span class="title">anonymous</span>(<span class="params">x</span>) </span>&#123;</span><br><span class="line">  <span class="keyword">if</span> (x &lt; <span class="number">0</span>) <span class="keyword">return</span></span><br><span class="line">  anonymous(x - <span class="number">1</span>)</span><br><span class="line">  <span class="comment">// arguments.callee(x - 1) // 开启严格模式则会报错</span></span><br><span class="line">&#125;)(<span class="number">100</span>)</span><br></pre></td></tr></table></figure>



<h2 id="作用域中的变量提升"><a href="#作用域中的变量提升" class="headerlink" title="作用域中的变量提升"></a>作用域中的变量提升</h2><p>作用域链查找原则：首先会在当前作用域中查找，如果没有的话会沿着作用域链向上查找， 直至全局作用域</p>
<ol>
<li>在全局作用域获取不到，报错：<code> ... is not defined</code></li>
<li>如果是赋值语句，就相当于给全局作用域添加了这样一个属性名和属性值</li>
</ol>
<h3 id="全局变量"><a href="#全局变量" class="headerlink" title="全局变量"></a>全局变量</h3><blockquote>
<p>在全局作用域（默认会提供一个最大的 window 对象）中声明的变量</p>
</blockquote>
<ul>
<li>函数没有形参，且函数体里没有 var 声明</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = <span class="number">6</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(a);</span><br><span class="line">    a = <span class="number">1</span>;</span><br><span class="line">&#125;</span><br><span class="line">fn();</span><br><span class="line"><span class="built_in">console</span>.log(a);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 6</span></span><br><span class="line"><span class="comment">// 1</span></span><br></pre></td></tr></table></figure>

<h3 id="私有变量"><a href="#私有变量" class="headerlink" title="私有变量"></a>私有变量</h3><blockquote>
<p>函数执行的时候形成的作用域是私有的，保护里面的变量不受外界干扰</p>
</blockquote>
<ul>
<li><p>形参</p>
<p>函数有形参，但没有传实参，且打印在赋值之前</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = <span class="number">6</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params">a</span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(a);</span><br><span class="line">    a = <span class="number">1</span>;</span><br><span class="line">&#125;</span><br><span class="line">fn();</span><br><span class="line"><span class="built_in">console</span>.log(a);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// undefined</span></span><br><span class="line"><span class="comment">// 6</span></span><br></pre></td></tr></table></figure>
</li>
<li><p>在私有作用域中声明的变量</p>
<p>函数没有形参，函数体里有 var 声明</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">  <span class="built_in">console</span>.log(a, b);</span><br><span class="line">  <span class="keyword">var</span> a = <span class="number">12</span>,</span><br><span class="line">      b = <span class="number">12</span>;</span><br><span class="line">  <span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">      <span class="comment">// 变量提升 var a；a 是私有的， b是 windows 全局作用域</span></span><br><span class="line">      <span class="built_in">console</span>.log(a, b);</span><br><span class="line">      <span class="comment">// 下面语句相当于 b=13; var a=b</span></span><br><span class="line">      <span class="keyword">var</span> a = b = <span class="number">13</span>;</span><br><span class="line">      <span class="built_in">console</span>.log(a, b);</span><br><span class="line">  &#125;</span><br><span class="line">  fn();</span><br><span class="line"><span class="built_in">console</span>.log(a, b);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// undefined undefined</span></span><br><span class="line"><span class="comment">// undefined 12</span></span><br><span class="line"><span class="comment">// 13 13</span></span><br><span class="line"><span class="comment">// 12 13</span></span><br></pre></td></tr></table></figure>
</li>
<li><p>复合（形参 + 私有作用域中声明变量）</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">    私有作用域 fn(a)</span></span><br><span class="line"><span class="comment">    1.形参赋值 a=12; a是私有的</span></span><br><span class="line"><span class="comment">    2.变量提升 var b; b是私有的</span></span><br><span class="line"><span class="comment">        只有 c 是全局作用域的</span></span><br><span class="line"><span class="comment">    3.代码自上而下执行</span></span><br><span class="line"><span class="comment">*/</span></span><br><span class="line"><span class="keyword">var</span> a = <span class="number">12</span>,</span><br><span class="line">    b = <span class="number">13</span>,</span><br><span class="line">    c = <span class="number">14</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params">a</span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(a, b, c);</span><br><span class="line">    <span class="keyword">var</span> b = c = a = <span class="number">20</span>;</span><br><span class="line">    <span class="built_in">console</span>.log(a, b, c);</span><br><span class="line">&#125;</span><br><span class="line">fn(a);</span><br><span class="line"><span class="built_in">console</span>.log(a, b, c);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 12 undefined 14</span></span><br><span class="line"><span class="comment">// 20 20 20</span></span><br><span class="line"><span class="comment">// 12 13 20</span></span><br></pre></td></tr></table></figure>

</li>
</ul>
<h3 id="块级作用域"><a href="#块级作用域" class="headerlink" title="块级作用域"></a>块级作用域</h3><ul>
<li>let 声明的变量不进行变量提升不能重复声明</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> a = <span class="number">10</span>,</span><br><span class="line">    b = <span class="number">10</span>;</span><br><span class="line"><span class="keyword">let</span> fn = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="comment">// a 变成私有变量，这个私有作用域没有 b，需要向上级查找</span></span><br><span class="line">    <span class="keyword">let</span> a = b = <span class="number">20</span>;</span><br><span class="line">    <span class="built_in">console</span>.log(a, b);</span><br><span class="line">&#125;;</span><br><span class="line">fn();</span><br><span class="line"><span class="built_in">console</span>.log(a, b);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 20 20</span></span><br><span class="line"><span class="comment">// 10 20</span></span><br></pre></td></tr></table></figure>

<h3 id="引用类型数据问题"><a href="#引用类型数据问题" class="headerlink" title="引用类型数据问题"></a>引用类型数据问题</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">    1.变量提升：fn -&gt; 0x111(全局)；arr -&gt; 0x222(全局)</span></span><br><span class="line"><span class="comment">    2.代码自上而下执行，执行 fn(arr)</span></span><br><span class="line"><span class="comment">        1.形参赋值 arr -&gt; 0x222[12, 13]</span></span><br><span class="line"><span class="comment">        2.重新赋值 arr[0]=100 -&gt; 0x222[100, 13]</span></span><br><span class="line"><span class="comment">        3.arr=[100] -&gt; 0x333[100](函数fn中)</span></span><br><span class="line"><span class="comment">        4.arr[0]=0 -&gt; 0x333[0]</span></span><br><span class="line"><span class="comment">*/</span></span><br><span class="line"><span class="keyword">var</span> arr = [<span class="number">12</span>, <span class="number">13</span>];</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params">arr</span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(arr);</span><br><span class="line">    arr[<span class="number">0</span>] = <span class="number">100</span>;</span><br><span class="line">    arr = [<span class="number">100</span>];</span><br><span class="line">    arr[<span class="number">0</span>] = <span class="number">0</span>;</span><br><span class="line">    <span class="built_in">console</span>.log(arr);</span><br><span class="line">&#125;</span><br><span class="line">fn(arr);</span><br><span class="line"><span class="built_in">console</span>.log(arr);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// [12,13]</span></span><br><span class="line"><span class="comment">// [0]</span></span><br><span class="line"><span class="comment">// [100,13]</span></span><br></pre></td></tr></table></figure>



<h2 id="变量提升特殊性"><a href="#变量提升特殊性" class="headerlink" title="* 变量提升特殊性"></a>* 变量提升特殊性</h2><h3 id="判断条件"><a href="#判断条件" class="headerlink" title="* 判断条件"></a>* 判断条件</h3><blockquote>
<p><strong>不论判断条件是否成立，都会进行变量提升</strong></p>
</blockquote>
<h4 id="条件不成立"><a href="#条件不成立" class="headerlink" title="* 条件不成立"></a>* 条件不成立</h4><p>var：<strong>只声明不定义</strong></p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 不管条件是否成立，都会进行变量提升，var a</span></span><br><span class="line"><span class="built_in">console</span>.log(a);</span><br><span class="line"><span class="keyword">if</span> (<span class="literal">false</span>) &#123;</span><br><span class="line">    <span class="comment">// 条件不成立，无法赋值</span></span><br><span class="line">    <span class="keyword">var</span> a = <span class="number">5</span>;</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(a);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// undefined</span></span><br><span class="line"><span class="comment">// undefined</span></span><br></pre></td></tr></table></figure>

<p>function：</p>
<ul>
<li><p>在老版本浏览器中：声明+定义</p>
</li>
<li><p>在新版本浏览器中：<strong>只声明不定义</strong></p>
<p>可以把它理解成函数表达式 <code>var fn = function()&#123;&#125;</code> ，只声明 <code>var fn</code> </p>
<p><code>delete fn</code> 返回 false（<code>var 变量</code> 无法使用 delete 删除）</p>
</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 在新版本浏览器中，判断条件中的function相当于只声明未定义，所以undefined</span></span><br><span class="line"><span class="built_in">console</span>.log(fn);</span><br><span class="line"><span class="built_in">console</span>.log(fn());</span><br><span class="line"><span class="keyword">if</span> (<span class="literal">false</span>) &#123;</span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="string">&quot;lion&quot;</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(fn);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// undefined</span></span><br><span class="line"><span class="comment">// TypeError: fn is not a function</span></span><br></pre></td></tr></table></figure>

<h4 id="条件成立"><a href="#条件成立" class="headerlink" title="条件成立"></a>条件成立</h4><ul>
<li><strong>判断条件成立，会对执行体中的 fn 进行变量提升</strong>（声明+赋值）</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(fn);</span><br><span class="line"><span class="keyword">if</span> (<span class="literal">true</span>) &#123;</span><br><span class="line">    <span class="built_in">console</span>.log(fn);</span><br><span class="line">    <span class="comment">// 全局作用域没有 fn ，给 fn 进行赋值</span></span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="string">&quot;hello&quot;</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(fn);</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// undefined</span></span><br><span class="line"><span class="comment">// [Function: fn]</span></span><br><span class="line"><span class="comment">// [Function: fn]</span></span><br></pre></td></tr></table></figure>

<h4 id="特殊情况"><a href="#特殊情况" class="headerlink" title="特殊情况"></a>特殊情况</h4><ul>
<li><p><strong>判断条件成立，如果有 function 定义的变量，在这个 function 函数后面更改变量的值，更改的都是私有变量</strong></p>
<p>可以把<code>if()&#123;&#125; 的 </code> <code>&#123; &#125;</code> 理解成块级作用域（特例： function(){} 的 <code>&#123; &#125;</code>是私有作用域）</p>
</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = <span class="number">0</span>;</span><br><span class="line"><span class="keyword">if</span> (<span class="literal">true</span>) &#123;</span><br><span class="line">    <span class="built_in">console</span>.log(a);  <span class="comment">// [Function: a]</span></span><br><span class="line">    a = <span class="number">5</span>;</span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">a</span>(<span class="params"></span>) </span>&#123;&#125;</span><br><span class="line">    a = <span class="number">7</span>;</span><br><span class="line">    <span class="built_in">console</span>.log(a);  <span class="comment">// 7</span></span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(a);  <span class="comment">// 5</span></span><br></pre></td></tr></table></figure>

<h3 id="自执行函数"><a href="#自执行函数" class="headerlink" title="自执行函数"></a>自执行函数</h3><ul>
<li><strong>自执行函数在当前作用域下不进行变量提升</strong></li>
</ul>
<p>新版浏览器：</p>
<blockquote>
<ol>
<li>在全局作用域中，没有变量提升</li>
<li>代码自上而下执行，<code>window.f = function()&#123;&#125;</code> 和 <code>window.g = function()&#123;&#125;</code></li>
<li>进入自执行函数，走到 if 语句中，函数 g 声明提升，此时 g 只声明未定义，相当于 undefined，所以 g() 会报类型错误</li>
</ol>
</blockquote>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">f = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line">&#125;;</span><br><span class="line">g = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line">&#125;;</span><br><span class="line">~(<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (g() &amp;&amp; [] == ![]) &#123;</span><br><span class="line">        f = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">            <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line">        &#125;;</span><br><span class="line">        <span class="function"><span class="keyword">function</span> <span class="title">g</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">            <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;)();</span><br><span class="line"><span class="built_in">console</span>.log(f());</span><br><span class="line"><span class="built_in">console</span>.log(g());</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// TypeError: g is not a function</span></span><br></pre></td></tr></table></figure>

<p>老版浏览器：</p>
<blockquote>
<ol>
<li><p>在全局作用域中，没有变量提升</p>
</li>
<li><p>代码自上而下执行，<code>window.f = function()&#123;&#125;</code> 和 <code>window.g = function()&#123;&#125;</code></p>
</li>
<li><p>进入自执行函数，走到 if 语句中，函数 g 声明提升并定义，</p>
<p>g() 为 true，参考下图优先级顺序， <code>==</code> 优先级高于 <code>&amp;&amp;</code> </p>
<p><code>g() &amp;&amp; [] ==  ![]</code> ，![] 转换为 false，再转换为0；[] 转换为 0；<code>[] == ![]</code> 返回 true</p>
<p><code>true &amp;&amp; true</code> 返回 true，进入循环</p>
</li>
<li><p>f 进行重新赋值，f 指向 <code>function()&#123;return false&#125;</code>，g 已经声明，不会再重复声明， g没有被修改</p>
</li>
<li><p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// false</span></span><br><span class="line"><span class="comment">// false</span></span><br></pre></td></tr></table></figure>
</li>
</ol>
</blockquote>
<p><img data-src="https://gitee.com/lilyn/pic/raw/master/js-img/JavaScript%E8%BF%90%E7%AE%97%E7%AC%A6%E4%BC%98%E5%85%88%E7%BA%A7.jpg" alt="JavaScript运算符优先级"></p>
<h3 id="赋值运算"><a href="#赋值运算" class="headerlink" title="赋值运算"></a>赋值运算</h3><ul>
<li><strong>只对等号左边进行变量提升</strong>（函数表达式）</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">console</span>.log(fn);</span><br><span class="line"><span class="built_in">console</span>.log(fn(<span class="number">1</span>, <span class="number">2</span>));</span><br><span class="line"><span class="keyword">var</span> fn = <span class="function"><span class="keyword">function</span> (<span class="params">n, m</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> n + m;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="built_in">console</span>.log(fn(<span class="number">3</span>, <span class="number">4</span>));</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// undefined</span></span><br><span class="line"><span class="comment">// TypeError: fn is not a function</span></span><br></pre></td></tr></table></figure>

<h3 id="return-语句"><a href="#return-语句" class="headerlink" title="return 语句"></a>return 语句</h3><ul>
<li>return 下面的代码虽然不能执行，但是可以进行变量提升（f2 进行变量提升）</li>
<li>return 后面的代码不能进行变量提升（f1 不进行变量提升）</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(f2);</span><br><span class="line">    <span class="built_in">console</span>.log(f1);</span><br><span class="line">    <span class="keyword">return</span> <span class="function"><span class="keyword">function</span> <span class="title">f1</span>(<span class="params"></span>) </span>&#123;&#125;;</span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">f2</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="string">&quot;bird&quot;</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line">fn();</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// [Function: f2]</span></span><br><span class="line"><span class="comment">// ReferenceError: f1 is not defined</span></span><br></pre></td></tr></table></figure>

<h3 id="重复变量名"><a href="#重复变量名" class="headerlink" title="重复变量名"></a>重复变量名</h3><ul>
<li>var 不会进行重复声明，但会重新赋值</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> num = <span class="number">4</span>;</span><br><span class="line"><span class="keyword">var</span> num = <span class="number">5</span>;</span><br><span class="line"><span class="built_in">console</span>.log(num); <span class="comment">// 5</span></span><br></pre></td></tr></table></figure>

<ul>
<li>function 在变量提升阶段 声明和定义是一同完成的，如果遇到重复声明定义的，会重新进行赋值</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">1.变量提升：</span></span><br><span class="line"><span class="comment">    function fn = 0x111</span></span><br><span class="line"><span class="comment">                = 0x222</span></span><br><span class="line"><span class="comment">                = 0x333</span></span><br><span class="line"><span class="comment">                = 0x444</span></span><br><span class="line"><span class="comment">2.代码从上到下执行</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line">fn();</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="number">1</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="number">2</span>);</span><br><span class="line">&#125;</span><br><span class="line">fn();</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="number">3</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// fn=100 给fn重新赋值</span></span><br><span class="line">fn = <span class="number">100</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="number">4</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// function 声明和定义早已完成， 100()则会报错</span></span><br><span class="line">fn();</span><br></pre></td></tr></table></figure>

<p>答案：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 4</span></span><br><span class="line"><span class="comment">// 4</span></span><br><span class="line"><span class="comment">// TypeError: fn is not a function</span></span><br></pre></td></tr></table></figure>
    </div>

    
    
    

	<div>
		  
			
<div class="my_post_copyright clearfix">
  <script src="//cdn.bootcss.com/clipboard.js/1.5.10/clipboard.min.js"></script>
  
  <!-- JS库 sweetalert 可修改路径 -->
  <script src="https://cdn.bootcss.com/jquery/2.0.0/jquery.min.js"></script>
  <script src="https://unpkg.com/sweetalert/dist/sweetalert.min.js"></script>
  <article class="box-left">
    <img src="https://gitee.com/lilyn/pic/raw/master/logo.jpg">
  </article>
  <article class="box-right">
    <p><span>本文标题:</span><a href="/posts/ea26882a/">JS 变量提升</a></p>
    <p><span>文章作者:</span><a href="/" title="访问 LiLyn 的个人博客">LiLyn</a></p>
    <p><span>原始链接:</span><a href="/posts/ea26882a/" title="JS 变量提升">http://llmysnow.top/posts/ea26882a/</a>
    <span class="copy-path"  title="点击复制文章链接"><i class="fa fa-clipboard" data-clipboard-text="http://llmysnow.top/posts/ea26882a/"  aria-label="复制成功！"></i></span>
  </p>
    <p><span>许可协议:</span>转载请保留原文链接及作者。</p>  
  </article>
</div>
<script> 
    var clipboard = new Clipboard('.fa-clipboard');
    $(".fa-clipboard").click(function(){
      clipboard.on('success', function(){
        swal({   
          title: "",   
          text: '复制成功',
          icon: "success", 
          showConfirmButton: true
          });
    });
    });  
</script>

		  
	</div>

      <footer class="post-footer">
          <div class="post-tags">
              <a href="/tags/JavaScript/" rel="tag"><i class="fa fa-tag"></i> JavaScript</a>
              <a href="/tags/%E5%89%8D%E7%AB%AF/" rel="tag"><i class="fa fa-tag"></i> 前端</a>
              <a href="/tags/%E4%BD%9C%E7%94%A8%E5%9F%9F/" rel="tag"><i class="fa fa-tag"></i> 作用域</a>
              <a href="/tags/%E5%8F%98%E9%87%8F%E6%8F%90%E5%8D%87%E7%89%B9%E6%AE%8A%E6%80%A7/" rel="tag"><i class="fa fa-tag"></i> 变量提升特殊性</a>
          </div>
		 
		<!--post-tags 彩色标签-->
		<script type="text/javascript">
		var tagsall=document.getElementsByClassName("post-tags")
		for (var i = tagsall.length - 1; i >= 0; i--){
			var tags=tagsall[i].getElementsByTagName("a");
			for (var j = tags.length - 1; j >= 0; j--) {
				var r=Math.floor(Math.random()*75+130);
				var g=Math.floor(Math.random()*75+100);
				var b=Math.floor(Math.random()*75+80);
				tags[j].style.background = "rgb("+r+","+g+","+b+")";
				tags[j].style.color = "#fff";
				tags[j].style.padding = "0 10px";
			}
		}                        
		</script>

        


        
    <div class="post-nav">
      <div class="post-nav-item">
    <a href="/posts/bf596df5/" rel="prev" title="牛客网本地刷题环境部署">
      <i class="fa fa-chevron-left"></i> 牛客网本地刷题环境部署
    </a></div>
      <div class="post-nav-item">
    <a href="/posts/5a7a946b/" rel="next" title="Python 获取文件夹下所有图片尺寸">
      Python 获取文件夹下所有图片尺寸 <i class="fa fa-chevron-right"></i>
    </a></div>
    </div>
      </footer>
    
  </article>
  
  
  



          </div>
          

<script>
  window.addEventListener('tabs:register', () => {
    let { activeClass } = CONFIG.comments;
    if (CONFIG.comments.storage) {
      activeClass = localStorage.getItem('comments_active') || activeClass;
    }
    if (activeClass) {
      let activeTab = document.querySelector(`a[href="#comment-${activeClass}"]`);
      if (activeTab) {
        activeTab.click();
      }
    }
  });
  if (CONFIG.comments.storage) {
    window.addEventListener('tabs:click', event => {
      if (!event.target.matches('.tabs-comment .tab-content .tab-pane')) return;
      let commentClass = event.target.classList[1];
      localStorage.setItem('comments_active', commentClass);
    });
  }
</script>

        </div>
          
  
  <div class="toggle sidebar-toggle">
    <span class="toggle-line toggle-line-first"></span>
    <span class="toggle-line toggle-line-middle"></span>
    <span class="toggle-line toggle-line-last"></span>
  </div>

  <aside class="sidebar">
    <div class="sidebar-inner">

      <ul class="sidebar-nav motion-element">
        <li class="sidebar-nav-toc">
          文章目录
        </li>
        <li class="sidebar-nav-overview">
          站点概览
        </li>
      </ul>

      <!--noindex-->
      <div class="post-toc-wrap sidebar-panel">
          <div class="post-toc motion-element"><ol class="nav"><li class="nav-item nav-level-2"><a class="nav-link" href="#%E5%8C%BF%E5%90%8D%E5%87%BD%E6%95%B0%E5%85%B7%E5%90%8D%E5%8C%96"><span class="nav-text">匿名函数具名化</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#%E4%BD%9C%E7%94%A8%E5%9F%9F%E4%B8%AD%E7%9A%84%E5%8F%98%E9%87%8F%E6%8F%90%E5%8D%87"><span class="nav-text">作用域中的变量提升</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#%E5%85%A8%E5%B1%80%E5%8F%98%E9%87%8F"><span class="nav-text">全局变量</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#%E7%A7%81%E6%9C%89%E5%8F%98%E9%87%8F"><span class="nav-text">私有变量</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#%E5%9D%97%E7%BA%A7%E4%BD%9C%E7%94%A8%E5%9F%9F"><span class="nav-text">块级作用域</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#%E5%BC%95%E7%94%A8%E7%B1%BB%E5%9E%8B%E6%95%B0%E6%8D%AE%E9%97%AE%E9%A2%98"><span class="nav-text">引用类型数据问题</span></a></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#%E5%8F%98%E9%87%8F%E6%8F%90%E5%8D%87%E7%89%B9%E6%AE%8A%E6%80%A7"><span class="nav-text">* 变量提升特殊性</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#%E5%88%A4%E6%96%AD%E6%9D%A1%E4%BB%B6"><span class="nav-text">* 判断条件</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#%E6%9D%A1%E4%BB%B6%E4%B8%8D%E6%88%90%E7%AB%8B"><span class="nav-text">* 条件不成立</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#%E6%9D%A1%E4%BB%B6%E6%88%90%E7%AB%8B"><span class="nav-text">条件成立</span></a></li><li class="nav-item nav-level-4"><a class="nav-link" href="#%E7%89%B9%E6%AE%8A%E6%83%85%E5%86%B5"><span class="nav-text">特殊情况</span></a></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#%E8%87%AA%E6%89%A7%E8%A1%8C%E5%87%BD%E6%95%B0"><span class="nav-text">自执行函数</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#%E8%B5%8B%E5%80%BC%E8%BF%90%E7%AE%97"><span class="nav-text">赋值运算</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#return-%E8%AF%AD%E5%8F%A5"><span class="nav-text">return 语句</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#%E9%87%8D%E5%A4%8D%E5%8F%98%E9%87%8F%E5%90%8D"><span class="nav-text">重复变量名</span></a></li></ol></li></ol></div>
      </div>
      <!--/noindex-->

      <div class="site-overview-wrap sidebar-panel">
        <div class="site-author motion-element" itemprop="author" itemscope itemtype="http://schema.org/Person">
    <img class="site-author-image" itemprop="image" alt="LiLyn"
      src="/images/logo.jpg">
  <p class="site-author-name" itemprop="name">LiLyn</p>
  <div class="site-description" itemprop="description"></div>
</div>
<div class="site-state-wrap motion-element">
  <nav class="site-state">
      <div class="site-state-item site-state-posts">
          <a href="/archives/">
        
          <span class="site-state-item-count">70</span>
          <span class="site-state-item-name">日志</span>
        </a>
      </div>
      <div class="site-state-item site-state-categories">
            <a href="/categories/">
          
        <span class="site-state-item-count">10</span>
        <span class="site-state-item-name">分类</span></a>
      </div>
      <div class="site-state-item site-state-tags">
            <a href="/tags/">
          
        <span class="site-state-item-count">162</span>
        <span class="site-state-item-name">标签</span></a>
      </div>
  </nav>
</div>
  <div class="links-of-author motion-element">
      <span class="links-of-author-item">
        <a href="https://github.com/llwodexue" title="GitHub → https:&#x2F;&#x2F;github.com&#x2F;llwodexue" rel="noopener" target="_blank"><i class="fab fa-github fa-fw"></i>GitHub</a>
      </span>
      <span class="links-of-author-item">
        <a href="https://blog.csdn.net/qq_38689395" title="CSDN → https:&#x2F;&#x2F;blog.csdn.net&#x2F;qq_38689395" rel="noopener" target="_blank"><i class="fas fa-copyright fa-fw"></i>CSDN</a>
      </span>
  </div>


  <div class="links-of-blogroll motion-element">
    <div class="links-of-blogroll-title"><i class="fa fa-link fa-fw"></i>
      友情链接
    </div>
    <ul class="links-of-blogroll-list">
        <li class="links-of-blogroll-item">
          <a href="https://scx69.gitee.io/blogs/" title="https:&#x2F;&#x2F;scx69.gitee.io&#x2F;blogs&#x2F;" rel="noopener" target="_blank">花开满城</a>
        </li>
        <li class="links-of-blogroll-item">
          <a href="http://yixin-oss.gitee.io/daytoy/" title="http:&#x2F;&#x2F;yixin-oss.gitee.io&#x2F;daytoy&#x2F;" rel="noopener" target="_blank">daytoy</a>
        </li>
    </ul>
  </div>

      </div>

    </div>
  </aside>
  <div id="sidebar-dimmer"></div>


      </div>
    </main>

    <footer class="footer">
      <div class="footer-inner">
        

        

<div class="copyright">
  
  &copy; 
  <span itemprop="copyrightYear">2022</span>
  <span class="with-love">
    <i class="fa fa-heart"></i>
  </span>
  <span class="author" itemprop="copyrightHolder">LiLyn</span>
    <span class="post-meta-divider">|</span>
    <span class="post-meta-item-icon">
      <i class="fa fa-chart-area"></i>
    </span>
    <span title="站点总字数">463k</span>
    <span class="post-meta-divider">|</span>
    <span class="post-meta-item-icon">
      <i class="fa fa-coffee"></i>
    </span>
    <span title="站点阅读时长">7:01</span>
</div>

<!--
  <div class="powered-by">由 <a href="https://hexo.io/" class="theme-link" rel="noopener" target="_blank">Hexo</a> & <a href="https://theme-next.org/" class="theme-link" rel="noopener" target="_blank">NexT.Gemini</a> 强力驱动
  </div>-->

<!--
<script async src="//busuanzi.ibruce.info/busuanzi/2.3/busuanzi.pure.mini.js"></script>
<div class="powered-by">
  <i class="fa fa-user-md"></i>
  <span id="busuanzi_container_site_pv">
    本站访问量:<span id="busuanzi_value_site_pv"></span>次
  </span>
  <span class="post-meta-divider">|</span>
  <span id="busuanzi_container_site_uv">
    本站总访客量：<span id="busuanzi_value_site_uv"></span>人
  </span>
</div>-->

        








      </div>
    </footer>
  </div>

  
  <script src="/lib/anime.min.js"></script>
  <script src="//cdn.jsdelivr.net/npm/lozad@1/dist/lozad.min.js"></script>
  <script src="/lib/velocity/velocity.min.js"></script>
  <script src="/lib/velocity/velocity.ui.min.js"></script>

<script src="/js/utils.js"></script>

<script src="/js/motion.js"></script>


<script src="/js/schemes/pisces.js"></script>


<script src="/js/next-boot.js"></script>




  
  <script>
    (function(){
      var canonicalURL, curProtocol;
      //Get the <link> tag
      var x=document.getElementsByTagName("link");
		//Find the last canonical URL
		if(x.length > 0){
			for (i=0;i<x.length;i++){
				if(x[i].rel.toLowerCase() == 'canonical' && x[i].href){
					canonicalURL=x[i].href;
				}
			}
		}
    //Get protocol
	    if (!canonicalURL){
	    	curProtocol = window.location.protocol.split(':')[0];
	    }
	    else{
	    	curProtocol = canonicalURL.split(':')[0];
	    }
      //Get current URL if the canonical URL does not exist
	    if (!canonicalURL) canonicalURL = window.location.href;
	    //Assign script content. Replace current URL with the canonical URL
      !function(){var e=/([http|https]:\/\/[a-zA-Z0-9\_\.]+\.baidu\.com)/gi,r=canonicalURL,t=document.referrer;if(!e.test(r)){var n=(String(curProtocol).toLowerCase() === 'https')?"https://sp0.baidu.com/9_Q4simg2RQJ8t7jm9iCKT-xh_/s.gif":"//api.share.baidu.com/s.gif";t?(n+="?r="+encodeURIComponent(document.referrer),r&&(n+="&l="+r)):r&&(n+="?l="+r);var i=new Image;i.src=n}}(window);})();
  </script>




  
<script src="/js/local-search.js"></script>













  

  

  <!-- 动态线条背景 -->
  <script type="text/javascript" src="//cdn.bootcss.com/canvas-nest.js/1.0.0/canvas-nest.min.js"></script>
  <!-- 页面点击小红心 -->
  <script type="text/javascript">
	!function(e,t,a){function n(){c(".heart{width: 10px;height: 10px;position: fixed;background: #f00;transform: rotate(45deg);-webkit-transform: rotate(45deg);-moz-transform: rotate(45deg);}.heart:after,.heart:before{content: '';width: inherit;height: inherit;background: inherit;border-radius: 50%;-webkit-border-radius: 50%;-moz-border-radius: 50%;position: fixed;}.heart:after{top: -5px;}.heart:before{left: -5px;}"),o(),r()}function r(){for(var e=0;e<d.length;e++)d[e].alpha<=0?(t.body.removeChild(d[e].el),d.splice(e,1)):(d[e].y--,d[e].scale+=.004,d[e].alpha-=.013,d[e].el.style.cssText="left:"+d[e].x+"px;top:"+d[e].y+"px;opacity:"+d[e].alpha+";transform:scale("+d[e].scale+","+d[e].scale+") rotate(45deg);background:"+d[e].color+";z-index:99999");requestAnimationFrame(r)}function o(){var t="function"==typeof e.onclick&&e.onclick;e.onclick=function(e){t&&t(),i(e)}}function i(e){var a=t.createElement("div");a.className="heart",d.push({el:a,x:e.clientX-5,y:e.clientY-5,scale:1,alpha:1,color:s()}),t.body.appendChild(a)}function c(e){var a=t.createElement("style");a.type="text/css";try{a.appendChild(t.createTextNode(e))}catch(t){a.styleSheet.cssText=e}t.getElementsByTagName("head")[0].appendChild(a)}function s(){return"rgb("+~~(255*Math.random())+","+~~(255*Math.random())+","+~~(255*Math.random())+")"}var d=[];e.requestAnimationFrame=function(){return e.requestAnimationFrame||e.webkitRequestAnimationFrame||e.mozRequestAnimationFrame||e.oRequestAnimationFrame||e.msRequestAnimationFrame||function(e){setTimeout(e,1e3/60)}}(),n()}(window,document);
  </script>
<script src="/live2dw/lib/L2Dwidget.min.js?094cbace49a39548bed64abff5988b05"></script><script>L2Dwidget.init({"pluginRootPath":"live2dw/","pluginJsPath":"lib/","pluginModelPath":"assets/","tagMode":false,"log":false,"model":{"jsonPath":"/live2dw/assets/hijiki.model.json"},"display":{"position":"right","width":150,"height":300},"mobile":{"show":true},"react":{"opacity":0.7}});</script></body>
</html>